Plasma accelerator



q m fia 'fi Q '5 U Fm Nov. 30, 1965 B. GOROWITZ ETAL 3,221,212

PLASMA ACCELERATOR Filed Oct. 27, 1961 2 Sheets-Sheet 1 INVENTOR 5.

PER GLOERSEN BY BERNARD GOROWITZ #1414 n KW AGENT Nov. 30, 1965 B. GOROWITZ ETAL 3,221,212

PLASMA ACCELERATOR Filed Oct. 2'7, 1961 2 Sheets-Sheet 2 F ig. 2

POTENTIAL SOURCE POTENTIAL 72 19; SOURCE I00 i- PULSE ,HIGH POWER SOURCE 70 J1 POTENTIAL SOURCE INVENTORS PER GLOERSEN BY BERNARD GOROWITZ AGENT United States Patent 0 3,221,212 RLASMA ACCELERATOR Bernard Gorowitz, Elkins Park, and Per Gloersen, Levittown, Pa, assignors to General Electric Company, a corporation of New York Filed Get. 27, 1961, Ser. No. 148,230 6 Ciaims. (Ci. 315-111) This invention pertains to the art of controllably producing and of accelerating discharges of gas plasma.

Increases in understanding of electrical discharges in gases have led to particular realization of two facts: that very high temperatures may exist in even small volumes of gases in a terrestrial environment, and that gas molecules in electrical discharges may exhibit very high velocities. So-called plasma torches which employ a jet of highly ionized gas plasma as a source of high-temperature energy are known; and there have, of late, been many proposals for the use of high-velocity plasma jets to provide force reactions for the maneuvering of space vehicles, as well as for terrestrial uses to simulate various effects of nature.

In general terms, gas plasma is produced by applying an electrical field to gas to produce an electrical discharge therein. The energy applied to the gas may either be used to produce uncoordinated molecular motion, or in creased temperature, or to produce coordinated molecular motion, or macroscopic velocity. For many uses of gas plasma torches or of jets it is desirable or necessary to control the time and the length of the discharge. If, as is frequently true, the plasma is discharged against a very low back pressure, the electric field provided to ionize the gas may be made insufiicient to produce any kind of discharge in the low concentration of gas existing at prevailing back pressure; in such case, the time of discharge may be determined by introducing a quantity of gas at higher pressure into the electric field at the desired time of discharge; and the duration of the discharge may be determined by the quantity of gas introduced. An alternative means of control is to regulate the time and duration of application of the electrical field which produces the discharge, letting the gas flow through the discharge space at all times. Both of these approaches have certain limitations, although not insuperable ones. Controlling the flow of gas requires moving parts, which require careful construction if they are to be reliable and require most ingenious design for high-speed operation; and controlling the high voltages and currents required for a heavy plasma discharge requires expensive switching equipment.

We have invented a way of controlling the time of occurrence and the duration of a plasma discharge without the necessity of controlling the gas flow correspondingly and without the necessity of switching the highpower source required to produce a high-energy electrical discharge through the plasma. We provide preliminary ionizing control means in the vicinity of the gas stream before its entrance into the region of the main electric field, which is made insufficient in magnitude to initiate ionization.

The preliminary control employs (in one embodiment) injector or ionizing electrodes with spacing between them such that (by Paschens law) an ionizing discharge may be caused in the volume of gas between them by a rela tively low voltage. Continuing flow thence carries the ionized gas into the region of the main electric field, which is of such intensity as to produce a discharge through the gas only when the gas fed in from the preliminary control means is already ionized. Given a fixed rate of gas flow, it is evident that there will be a determinate relation between the length of time the ionizing 3,221,212 Patented Nov. 30, 1965 discharge is passed through the gas at the preliminary control means and the length of time that the thereby ionized gas produces a discharge in the main electric field. That is, the longer the ionizing discharge is applied to produce ionized gas in the preliminary control means, the longer it will take the gas thus ionized to pass through the main field region. It is, of course, possible and desirable for reasons of economy to turn off the flow of gas during long periods when it is known that no discharge of plasma will be required; and automatic means may be employed to turn on the gas before a discharge is initiated and to turn it off after a sequence of discharges has been completed. However, the temperature of the unionized gas will be relatively low and, because its volume will be small at relatively low temperature, its velocity will also be relatively low. Thus both the thermal effect and the momentum will be negligible in the absence of an electrical discharge; and the control achieved by low-power electrical switching will be equivalent in all desirable respects to that produced either by gas valving or by high-power electrical switching. The rate of gas flow may, of course, also be controlled to give an additional control parameter.

The electrical discharge produced by the main field will increase the total ionization over that originally produced to trigger the discharge. There is thus an increase in ionization which may be regarded as occurring beyond a first stage (the preliminary control means) at a second stage (the main field region). The application of our invention need not be confined to two stages; the output of the second stage may be fed into a third region in which an electrical field is maintained inadequate, like that of the second stage, to produce an electrical discharge in the gas normally present unless ionized gas is caused to be present by some auxiliary means, but the field in the third region is produced by a source of such impedance that, when a discharge has been initiated by the introduction of ionized gas from the output of the second stage, energy exceeding that of the second stage discharge will be liberated. It might be supposed that the cascading of stages as described would lead to instability, since it might appear that the second stage must inevitably trigger itself spontaneously on occasion, in the absence of a first stage discharge; but experiment has shown a surprising stability in that, during nearly two million cycles of operation of a system in which a 70-watt first stage triggered a 7-kilowatt second stage, there were no uncontrolled second-stage discharges, and no failures of the second stage to fire upon triggering by the first stage. It was also observed that, because the initial discharge occurs at relatively low power, the erosion of electrodes in the embodiment of our invention tends to be appreciably less than that observed when a single set of electrodes is pulsed at high power.

For the better understanding of our invention, we have provided figures of drawing in which FIGURE 1 represents pictorially, in section, an electrode structure suitable for two-stage operation in accord ance with our invention;

FIGURE 2 represents schematically the structure of FIGURE 1 connected electrically to function in accordance with our invention; and

FIGURE 3 represents partly in section and schematically an electrode structure suitable for the practice of our invention employing three stages connected electrically to function according to our invention.

Referring to FIGURE 1, it will be understood that there is represented only the lower half of a structure whose whole actually possesses circular cylindrical symmerty but is here represented as sectioned by a plane passing through the axis of symmetry. This is preferable for clarity because a simple unscctioned View would conceal the internal details. An end plate 12 of metal (half hard copper was used for most of the metal parts in a model actually constructed and used) bears a machine-d nozzle 14 which is fixed to plate 12 so that an entrance port 16 in plate 12 is in communication with a gas inlet chamber 18 which leads at the distal end to a plurality of orifices 20 which discharge into a first stage chamber 22. First stage chamber 22 is bounded by nozble 14, a tubular extension 24 of nozzle 14, and by a conical electrode 26, which is supported as represented by a plate 28, which is in turn supported by a pedestal tube 30 and a support plate 32. Concentricity of nozzle 14 and electrode 26, and all their associated conductive parts is maintained by the pull of plastic bolts 34, which draw support plate 32 toward end plate 12, the two plates being separated from and electrically insulated from each other by insulator 36, which was made of glass in the embodiment actually used. A gas-tight seal is provided between insulator 36 and plates 12 and 32 by O-ring gaskets 38 and 40. The concentric alignment of nozzle 14, extension 24, and electrode 26 leaves a space or vent between extension 24 and electrode 26 which leads into a second stage chamber 42 formed by extension 24 and a tubular electrode 44, which is supported by a metal plate 46 which is held in alignment with the other parts of the electrode system by insulating bolts 48 and insulating cylinder 56 (which may be of glass) which is sealed to plates 46 and 28 by gaskets or O-rings 52 and 54. To facilitate mounting of the entire assembly with no electrical contact through the mounting to any parts of the electrode system, there is provided an adapter sleeve 56, made of glass in the original, having a shoulder at the near end so that a clamp ring 58, hearing on a conical gasket 60 as represented, may hold the shoulder of sleeve 56 against plate 46 through the tension of insulating bolts 62, an O-ring gasket 64 being provided to insure a gastight seal between sleeve 56 and plate 46.

FIGURE 2 represents the embodiment of FIGURE 1 with a gas source 66 represented symbolically as a rectangle connected to entrance port 16 by a tube 68. End plate 12 is represented as connected to ground by a ground conductor 70, which also serves to ground a controllable trigger pulse source 72 and a continuous potential source 74. (The means for controlling controllable source 72 is not represented, since this will depend in nature upon the intended use of our invention, and the control of trigger pulse sources is part of the well known art.) Trigger pulse source 72 is connected by conductor 76 to plate 28, and continuous potential source 74 is, as represented, connected by conductor 78 to plate 46. In accordance with the description already given, gas source 66 is adjusted to give a flow of gas through tube 68 into the gas inlet chamber 18, whence the gas flows into first stage chamber 22 and thence into second stage chamber 42 and so out of the apparatus. When a trigger pulse is produced by source 72, this is applied via conductor 76 to plate 28, changing the potential of conical electrode 26 with respect to nozzle 14 and its extension 24, which are in electrical connection with end plate 12 and are thus at ground potential. This potential change produces a field which ionizes the volume of gas in first stage chamber 22, expanding it thermally and also, as a result of interaction between the cur-rent flowing through the gas and the magnetic field produced by the same current flowing in the electrode structure, accelerating it magnetically as well as thermally out through the space between conical electrode 26 and tubular extension 24. This ionized gas passes into the second stage chamber 42. The potential provided by source 74 is applied by conductor 78 to plate 46, which is in electrical connection with tubular electrode 44. Thus there is an electrical field in second stage chamber 42 between electrode 44 and tubular extension 24. The ejection of ionized gas into this field permits the field to produce a heavy electrical discharge which heats the gas and also accelerates it by reaction between the current in the gas and the magnetic field produced by the flow of the same current in electrodes 44 and 24. The combined electrical acceleration and temperature increase drives or expands the ionized plasma out through the open end of electrode 44 with high velocity. When the ionizing discharge in the first state chamber 22 ceases, as a result of the cessation of the pulse from source 72, and the discharge of plasma from the open end of electrode 44 has swept the ionized gases out of the field, the discharge ceases, the potential provided by source 74 being of such magnitude, as previously explained, as'will not sufiice to produce a discharge through the gas in the absence of an effective concentration of ions in the gas entering chamber 42. Thus the occurrence and the duration of plasma discharges from the apparatus will be controlled by the occurrence and duration of the electrical pulses from source 72.

FIGURE 3 represents schematically a section through a central plane of an electrode structure which preferably has circular cylindrical symmetry. The extreme mechanical detail which was portrayed in FIGURE 1 has not been given here because the complexity of the electrode structure of FIGURE 3 renders it desirable to eliminate all unnecessary distracting features. Thus electrodes which might conveniently actually be fabricated of several parts are shown as units since it is within the capacity of any ordinary metal worker to fabricate them thus. Ancillary devices are represented simply symbolically, since they are all well known objects. Gas from source 66 is fed by tube 68 to an unnumbered plenum, or gas inlet chamber in electrode 80, which is proveded with exit ports 82 leading from the plenum chamber. Electrode has an extension 84, beyond which there exists a second chamber 86 formed within a second electrode 88. Ports 90 lead from chamber 86 to a third chamber 92, which is formed by part of the exterior of electrode 88 and the interior of electrode 93. Chamber 92 is not provided with ports as such, but its open end discharges along the exterior of electrode 88, toward its distal end, inside electrode 94, into a space 96, which is open to the exterior at its distal end, formed by the termination of electrodes 88 and 94. The connection of the electrical auxiliaries to the electrodes is similar in purpose but slightly different in detail from that represented in FIGURE 2. Common ground conductor 70 is connected, as before, to sources 72 and 74; but it is connected also to electrode 88, and to high power continuous potential source 98. The ungrounded output of source 72 is connected by conductor 76 to electrode 80. The application of the output of source 72 to electrode 80 necessitates the electrode 80 not be grounded by its connection via tube 68 to gas source 66. The simplest way of accomplishing this is to make tube 63 of some non-conducting material such as an insulating plastic like polyethylene or rubber. The ungrounded output of source 74 is connected by conductor 78 to electrode 93; and the ungrounded output of high-power source 98 is connected by conductor 100 to electrode 94. In order to preserve the alignment of the various electrodes and to close off the spaces between them at their boundaries against gas leakage without connecting the separate electrodes electrically, they are represented as being separated by insulating collars 102, 104, and 106 as represented. These may be of glass, with O-ring gaskets not represented. The means of mechanical attachment of the various parts may be by an obvious appliction of plastic bolts, as represented in FIGURE 1, or by the use of suitable cement at their interfaces, (in which case O-ring gaskets may be omitted) or by any other conventional means. An external insulation sleeve similar to 56 of FIGURE 1 may be provide to surround electrode 94, but has been omitted for clarity.

The operation of the embodiment represented in FIG- URE 3 is as follows. Gas from source 66 passes through tube 68 into the plenum chamber of electrode 84 and out through ports 82. Thence it passes into chamber 86 and through ports 90. After leaving ports 90 it moves in chamber 92 past the distal end of electrode 93 into chamber 96 and thence is discharged at the extreme ends of electrode 94 and electrode 88, in the concentric space between them. During this time, potential from source 74 maintains an electric field in chamber 92 between electrode 93 and grounded electrode 88; but this field is of such magnitude that, in the absence of an effective concentration of ions in the gas emerging from ports 90, it does not produce an electrical discharge in chamber 92. Similarly, potential from source 98 maintains a field in chamber 96 between electrode 94 and grounded electrode 88; but this, too, is of such magnitude that it does not produce a discharge in chamber 96 if the gas entering the chamber does not include a supply of ions produced previous to the introduction of the gas into chamber 96. However, when a potential pulse is applied by controllable pulse source 72 to electrode 80, a field is produced between electrode 80 and grounded electrode 88 which causes an ionizing disruptive discharge through the gas, heating it and accelerating it magnetically into chamber 86 and causing gas, with a high concentration of ions, to discharge through ports 90 into chamber or space 92. The presence of the ions in chamber 92 permits the field produced by the potential of source 74 to cause an electrical discarge of much higher energy than that in space 86, thus increasing the ionization and the temperature of the gas originally ionized in chamber 86, and causing it to be discharged (with the additional assistance of electromagnetic forces) out of the end of electrode 93. It will be observed up to this point, the action described has been similar to that described in connection with FIGURE 2, and, indeed, if electrode 88 were terminated at the plane defined by the remote, or right-hand end of insulating collar 106, and electrode 94 were removed, the embodiment of FIGURE 3 would be a device similar in functioning and capabilities to that of FIGURE 2, discharging a moderately high-energy plasma burst into space. In fact, however, with the extension of electrode 88 as actually shown, and with electrode 94 maintained by source 98 at a potential such as to produce a field in space 96, this moderately high energy plasma burst is discharged into the field in space 96, which thereupon produces an extremely high-energy discharge in the plasma, increasing its energy content many times more, and causing it, by a combination of thermal expansion and electromagnetic drive as previously described to be discharged into space from the extreme or distal end of electrode 94. Cessation of the discharge in chamber 84 will, as in the operation described in connection with FIGURE 2, cause the ionized gas to be swept out of the apparatus and replaced by unionized gas, causing the various discharges to cease.

It is, of course, clear from the preceding descriptions that the embodiment of FIGURE 3 could be used to supply a plasma charge into yet another chamber where a field is maintained by yet another potential source of yet higher power output, and so ad infinitum in successive stages, with the limits set only by the practical problems of making such power available at the electrodes. However, a power ratio of 70 watts to 7 kilowatts, as obtained with the embodiment represented in FIGURES 1 and 2 is numerically equal to one hundred; and, even if this ratio were only approximated in a multiple stage device, the geometric increase in power suggests that for most purposes only a few stages would be required. If a convenient maximum discharge energy per state (as determined by electrode cooling, convenient supply of electrical power, or other considerations) were reached in a design without achieving the desired total energy input to the gas, successive maximum-energy stages could be used to add energy linearly with the number of added stages, and thus still achieve the benefits of our invention. It is also perhaps now more clear than in the initial discussion that our invention offers peculiar flexibility in controlling the length of the plasma discharge, in that the operations described in the various stages will continue so long as source 72 continues to provide ionizing discharges in the first or preliminary ionization or discharge chamber, assuming that the various other source such as 74 and 98 can continue to perform their required function for the required time duration, and that resultant electrode heating can be tolerated.

There are, of course, numerous obvious technical tricks which may be applied to the design and operation of devices in accordance with the teachings of our invention, some of which may be of particular use in given circumstances so nearly unique that to attempt to catalogue them would be pointless, particularly in a field so rich in technical knowledge as the present one. For example, the conventional electrodeless discharge may be employed in the preliminary or controlling ionization, in which case the chamber in which the initial ionization is produced might be of insulating material, surrounded with a coil to produce the high-frequency field required to generate the electrodeless discharge. For our purposes this particular scheme was not the preferred one, and, since it is an application of the teachings, it has not been represented here. Again, the electrodes need not have circular symmetry as represented in the specification, but the uniformity of field produced by circularly symmetrical electrodes caused us to prefer that structure over rectangular or other possible designs. Clearly, special circumstances might cause any of these, or many other modifications in the embodiment of our invention, to be preferred in a given case; but these may be produced by our teachings by ordinary skill.

Similarly, the power supplies 74 and 98 need not be simple D.-C. power supplies; they may vary conveniently be designed to give a heavy current for a limited time (at least as great as the longest desired duration of a single plasma discharge). Thus a relatively high-impeda'nce power supply having a condenser of suificient capacity in parallel with its output could, during the interval between plasma discharges, charge the condenser to the required potential. When ionized gas cause an electrical discharge to occur, the condenser would discharge its energy content at a very high rate, corresponding to a power rating far exceeding that of the charging source. Alternatively, a pulse-forming line such as the Guillemin voltage-fed line could be used instead of the condenser, if it were desired that the disicharge power be constant during a given discharge. These techniques (for which reference is made to Pulse Generators by Glasoe and Lebacqz, volume 5 of the Radiation Laboratory Series published by McGraw-Hill Book Company of New York, NY.) for providing short pulses of power far greater than the average power available are likely to be of use in the practice of our invention where the duty cycle is low and the required power during a pulse is very high-a condition under which the advantages of our invention would be particularly valuable. Again, however, the particular circumstances of use of our invention will require that one skilled in the art determine which form of power supply is most suitable; and so, in general.

What is claimed is:

1. In combination, a source of gas, means connected to the said source of gas to convey gas therefrom to a preliminary ionization space; controllable means for applying to said gas in said preliminary ionization space electric energy to ionize said gas, thereby causing said ionized gas to expand into a main ionization space; means defining said main ionization space and an exit therefrom; current pulse providing means producing in said ionization space an electric field insufiicient to produce an electrical discharge in said gas in the absence of preliminary ionization in the said gas and sulficient to produce in the 75 presence of preliminary ionization an electrical discharge of greater energy than the energy applied to said gas in said preliminary ionization space and thus to accelerate the said gas through the said exit.

2. A source of gas connected to pass gas through an electric field insufiiciently strong to produce an electrical discharge through the said gas in the absence of a substantial concentration of ions in the said gas upon its entrance into the said electric field and sufiiciently strong to produce an electrical discharge through the said gas in the presence of a substantial concentration of ions in the said gas upon its entrance into the said electric field; means for limiting the duration of the said electrical discharge to a pulse; and controllable means for producing a substantial concentration of ions in the said gas before its entrance into the said electric field.

3. A device for controlling the production of electrical discharges in a flowing gas which comprises a path for gas fiow, controllable means at a first position in the said path for creating in a first volume of the said gas pulses of electric field of such magnitude as to produce a disruptive ionizing electrical discharge in the said gas at the said first position, and at a second position further along the said path in the direction of gas flow current pulse providing means for applying to a second volume of the said gas an electric field insufiicient to ionize the gas in the said second volume when the gas enters the said second volume in a substantially unionized condition and sutficient to increase the ionization of the gas in the said second volume when the gas enters the said second volume containing ions produced in the said first volume.

4. The device claimed in claim 3, further characterized by the presence at a third position in the therein said path for gas fiow of current pulse providing means for applying to a third volume of the said gas an electric field insufiicient to produce an electrical discharge in the gas in the said third volume when the gas enters the said third volume in a substantially unionized condition and sufficient to increase the energy of the gas in the said third volume by electrical discharge therein when the gas enters the said third volume containing ions produced in it previous to its entry into the said third volume.

5. A system defining a path for gas flow from a source of said gas to a discharge station at the end of the said path; at least two opposed electrodes defining a first stage in the said path and connected to a first source of potential controllable in time to produce ionization in the gas at the said first stage; subsequent sets of opposed electrodes, each said set comprising at least two electrodes, defining successive stages along the said path, each said subsequent set being connected to a current pulse providing source of potential of such magnitude as to produce at the thereby defined successive stage an electric field insufiicient to produce an electrical discharge in the gas at such successive stage when the said gas flows into the said successive stage in an unionized condition and sufficient to produce an electrical discharge in the gas at such successive stage when the said gas flows into the said successive stage in an ionized condition.

6. A device for controllably producing discharges of gas plasma of high energy content responsively to controllable electrical discharges of low energy, comprising; a path for sequential gas flow; a first stage in the said path having means for producing controllable ionizing discharges of low energy in the gas; a second stage located farther along the said path in the direction of gas flow having current pulse providing means for applying to the said gas an electric field of magnitude insufficient to produce an electrical discharge in the said gas when the said gas enters the said second stage in a substantially unionized condition and sufficient to produce a high energy electrical discharge in the said gas when the said gas enters the said second stage in an ionized condition.

GEORGE N. WESTBY, Primary Examiner. 

1. IN COMBINATION, A SOURCE OF GAS, MEANS CONNECTED TO THE SAID SOURCE OF GAS TO CONVEY GAS THEREFROM TO A PRELIMINARY IONIZATION SPACE; CONTROLLABLE MEANS FOR APPLYING TO SAID GAS IN SAID PRELIMINARY IONIZATION SPACE ELCTRIC ENERGY TO IONIZE SAID GAS, THEREBY CAUSING SAID IONIZED GAS TO EXPAND INTO A MAIN IONIZATION SPACE; MEANS DEFINING SAID MAIN IONIZATION SPACE AND AN EXIT THEREFROM; CURRENT PULSE PROVIDING MEANS PRODUCING IN SAID IONIZATION SPACE AN ELECTRIC FIELD INSUFFICIENT TO PRODUCE AN ELECTRICAL DISCHARGE IN SAID GAS IN THE ABSENCE OF PRELIMINARY IONIZATION IN THE SAID GAS AND SUFFICIENT TO PRODUCE IN THE PRESENCE OF PRELIMINARY IONIZATION AN ELECTRICAL DISCHARGE OF GREATER ENERGY THAN THE ENERGY APPLIED TO SAID GAS IN SAID PRELIMINARY IONIZATION SPACE AND THUS TO ACCELERATE THE SAID GAS THROUGH THE SAID EXIT. 